Fig. 1. Responses of transpiration efficiency (TE) to CO₂ concentration in C₃ and C₄ plants: (A) relative increases in TE with increasing CO₂ concentration (normalized to 350 µmol mol^-1 CO₂), and (B) possible absolute responses of TE to CO₂. Transpiration efficiency was calculated assuming a ratio of intercellular to atmospheric CO₂ concentration of 0.7 for C₃ plants and 0.4 for C₄ species and a mole fraction water vapor gradient from the leaf to bulk air of 12 x 10^-3 mole mole^-1 across CO₂ concentrations.